In a WDM transmission system, two or more optical data carrying channels, each defined by a different carrier wavelength, are combined onto a common path for transmission to a remote receiver. The carrier wavelengths are sufficiently separated so that they do not overlap in the frequency domain. The multiplexed channels are demultiplexed at the receiver in the optical and possibly also in the electrical domain. Demultiplexing in the optical domain requires using frequency-selective components such as optical gratings or bandpass filters. Typically, in a long-haul optical fiber system, an optical amplifier would amplify the set of wavelength channels simultaneously, usually after traversing distances less than about 120 km.
One class of optical amplifiers is rare-earth doped optical amplifiers, which use rare-earth ions as the active element. The ions are doped in the fiber core and pumped optically to provide gain. The silica fiber core serves as the host medium for the ions. While many different rare-earth ions such as neodymium, praseodymium, ytterbium etc. can be used to provide gain in different portions of the spectrum, erbium-doped fiber amplifiers (EDFAs) have proven to be particularly attractive because they are operable in the spectral region where optical loss in the fiber is minimal. Also, the erbium-doped fiber amplifier is particularly useful because of its ability to amplify multiple wavelength channels without crosstalk penalty, even when operating deep in gain compression. EDFAs are also attractive because they are fiber devices and thus can be easily connected to telecommunications fiber with low loss.
FIG. 1 shows an energy level diagram for the Er+3 system in silica fiber. As shown, light of wavelength 980 nm is absorbed by the erbium ions, exciting the ions to the higher energy state 4I11/2. This excited state rapidly decays (with a time constant τ32 of about 10 microseconds) to the metastable state 4I13/2 without radiative emission. The metastable state alternatively may be reached by the absorption of light at 1480 nm, which corresponds to the upper edge of the band defining the metastable state. The metastable state deexcites by emitting photons at different wavelengths, with peak photon emission occurring at about 1530 nm. This deexcitation may occur spontaneously or by stimulated emission with an optical signal having a wavelength around 1530 nm. Since the metastable state is relatively long-lived (with a time constant τ21 of about 10 milliseconds), stimulated emission is much more likely to occur than spontaneous emission under typical operating conditions. Stimulated emission causes amplification of the optical signal that induced the stimulated emission.
The signal power directed to the input of an optical amplifier employed in an optical communication network can vary for a large number of reasons. For example, power variations can be caused by an intentional increase or decrease in the number of channels for the purpose of routing traffic, by the unintentional loss of channels due to a fiber cut or human error, changes in span losses, and component loss changes due to aging or temperature fluctuations. Additionally, in packet-switched networks, the traffic pattern is often fractured in time where there may be substantial time delays between packets transmitted over the network. Such networks see some periods of source inactivity followed by sudden periods of traffic bursts. Consequently, the network is prone to power fluctuations in time on the optical line. With a constant pump power, channels passing through an optical amplifier operating in saturation become coupled due to the population inversion and pump dynamics of the amplifier. When channels are dropped or added the power in those channels is transferred to or taken from the power of the remaining channels. Similarly, in packet-switched networks, the packets passing through the optical amplifier may be amplified to different power levels depending on the time spacing between packets, i.e., after a period of inactivity, the first packet through the amplifier will have a much larger power than the last packet through. Both types of events lead to power excursions on the optical channels that build at every node and may eventually surpass the nonlinearity limits or push the channel powers or optical signal to noise ratios out of the dynamic range of the downstream receivers.
Because of the aforementioned problems it is important to maintain a constant amplifier gain as the input power changes. This type of control is commonly referred to as automatic gain control (AGC) or transient control. It is well known that AGC can be achieved by adjusting the pump power supplied to the amplifier. In general, the required change in pump power depends not only on the input signal power level but also on the spectral content of the input signal. Most AGC algorithms consist of three parts: (i) detection of a change in the monitoring parameters; (ii) generation of an error signal; and (iii) modification of the control parameters to return the error signal to zero. With AGC the gain of the amplifier at a particular optical frequency, v, is held constant such that:                               a          .                                          ⁢                                                    P                out                            ⁡                              (                v                )                                                                    P                in                            ⁡                              (                v                )                                                    =                              G            ⁡                          (              v              )                                =                      const            .                                              (        1        )            where Pout(ν) is the output signal power at frequency (ν), Pin(ν) is the input signal power at frequency ν, and G(ν) is the gain at frequency ν.
A well-known technique for implementing AGC by controlling pump power is a feedback arrangement in which the parameters used to determine the appropriate pump power include the input and output optical signals, which are used to determine the actual gain of the amplifier. This measured gain may then be used to adjust the pump power until the desired gain is achieved. For example, FIG. 2 shows an optical amplifier with such a feedback control. The arrangement of FIG. 2 comprises an erbium doped fiber 1, a pump laser 2, a wavelength multiplexer 3 which multiplexes the pump laser output and an input optical signal which is to be amplified and is input at port 4, an input signal tap 12, which serves to split off a small portion of the input signal to doped fiber 1, an output signal tap 5, which serves to split off a small portion of the total output signal, an output port 6 for receiving the amplified optical signal, detectors 8 and 14, electronic amplifiers 9 and 16 and a feedback circuit 10.
In operation, the optical signal to be amplified is input via port 4 of multiplexer 3, multiplexed with the optical pump signal output from laser 2 and amplified in the erbium doped fiber 1. Tap 12, which may be a fused fiber coupler, for example, splits off a small proportion of the signal input to the fiber 1. This small part of the amplified signal, which is employed as a control signal, is detected by detector 14, amplified by electronic amplifier 16 and applied to the feedback circuit 10. Likewise, tap 5, which may also be a fused fiber coupler, for example, splits off a small proportion of the total output power from fiber 1. This small part of the output power, which also serves as a control signal, is detected by detector 8, amplified by amplifier 9 and applied to the feedback circuit 10. Feedback circuit 10 determines the amplifier gain based on the two control signals it receives. In some cases, an estimate of the ASE power generated by the amplifier for a given gain is subtracted from the total output power to more accurately determine the signal output power. The output from the feedback circuit 10 is applied to the pump laser 2 and serves to vary the pump laser 2 output power to maintain constant gain.
One variant of the feedback arrangement shown in FIG. 2 employs the amplified spontaneous emission (ASE) rather than the optical signal power itself as the control signal. As is well known, all optical amplifiers generate ASE. As shown in FIG. 3, conventional “C-band” erbium amplifiers provide substantial gain in the range of 1529–1564 nm. Likewise, the ASE is strongest over this same wavelength range because the ASE power is directly proportional to amplifier gain. That is, the intensity of the amplified spontaneous emission from the amplifier is dependent on amplifier gain, and thus, a measure of ASE provides an indirect measure of the amplifier gain. Accordingly, one or more wavelengths within the 1529–1564 nm range may be reserved for measuring ASE at that wavelength. For example, in FIG. 3, ASE is measured at a wavelength of 1.551 microns, which can therefore be used to form the basis of a gain control loop of the form illustrated in FIG. 4.
In FIGS. 2 and 4, like elements are denoted by like reference numerals. In FIG. 4, however, coupler 5 is now a wavelength selective coupler that splits off a small portion of the ASE. Thus, in this arrangement the amplifier gain is monitored by monitoring the ASE over the wavelengths demultiplexed by coupler 5, which is used by the feedback loop to keep the amplifier gain constant by varying the pump power accordingly.
The previously discussed feedback arrangement for providing an optical amplifier with AGC in which the ASE is used as the control signal has a significant disadvantage. In particular, this technique assumes that the power of the ASE scales with the amplifier gain for all signal input powers so that a constant ASE power implies a constant gain. Unfortunately, this is generally not the case for all signal input powers because of variations in the noise figure of the amplifier at different signal input powers. As a result, there may be a change in the amplifier gain even while the ASE remains constant.
Accordingly, there is a need for an optical amplifier having an improved automatic gain control arrangement where the ASE is used as the control signal.